In a speech at Microsoft's headquarters, Romney repeated calls for a new free-trade zone among countries that agree to respect those intellectual property rights and called for broader punishments for China if they don't allow their currency, the yuan, to appreciate in value.

Romney didn't offer any new proposals, instead using the speech to hit China's trade practices. He said China's currency manipulation has cost "millions of jobs" in the U.S.

"I want to make sure that people we trade with follow the rules and if someone consistently cheats, I want to make sure they understand that can't go on," he said.

China's government controls the value of its currency against the dollar, a practice that makes Chinese products cheaper in U.S. markets and U.S. goods more expensive in China. U.S. manufacturers say the currency manipulation hikes the price of American imports by as much as 40 percent.

Romney also harshly criticized China for ignoring U.S. copyright laws, accusing it of counterfeiting American designs, technology and pirating software.

The Chinese are "stealing ... intellectual property, appropriating it at no cost, duplicating and selling it around the world," he said. "There's also hacking going on, where Chinese companies or even the government itself, perhaps, hacks into computers to steal technology."

U.S. officials have long pointed to China as one of the leading safe havens for cybercriminals, or government-sponsored or tolerated hacking.

China has said that allegations of cyberespionage against U.S. companies were groundless, and the sources of Internet attacks are notoriously difficult to pin down.

Amid stubbornly high unemployment and a persistent economic recession, the GOP presidential candidates have often attacked China as they seek to convince voters they can create jobs and turn the economy around.

China's monetary policy has come under fire in Congress, too. The Senate this week passed a bill to impose higher tariffs on China if they don't allow their currency to gain value more quickly. That bill is unlikely to pass the House, where Republican leaders have opposed it.

Romney said if elected president he would immediately label China as a currency manipulator. He also wants American trade authorities to consider China's currency policies as a subsidy on Chinese goods, which would allow the U.S. to put a countervailing subsidy on Chinese imports.

"A year ago, Romney hit Obama in (Romney's) book for being too tough on China. Now Mitt's a trade warrior? Should he have called his book No Shame!" top Obama adviser David Axelrod wrote on Twitter on Thursday.

Romney rival Jon Huntsman, a former ambassador to China, has also criticized Romney's trade proposals. "I don't subscribe to the Don Trump school or the Mitt Romney school of international trade. I don't want to find ourselves in a trade war," Huntsman said during Tuesday's debate in New Hampshire.

Economists largely agree with Huntsman; the policy debate among Republicans — Democrats, too — is whether that risk is worth it. Opponents of confronting China worry about a trade war that the fragile global economy cannot afford.

China may have more to lose than the U.S. if trade in goods were curtailed. But Washington depends heavily on China to buy U.S. Treasury securities to help finance its budget deficits.

As part of his Thursday trip, Romney also held a fundraiser at a downtown Seattle hotel — and was confronted by demonstrators from the Occupy Seattle protest, one of the demonstrations across the country that have grown out of New York's Occupy Wall Street protests.